1. Field of the Invention
This invention relates to a process for operation of molten carbonate fuel cells, particularly to a heating process used during initial start-up of a single cell or cell stack to ensure high electrolyte saturation of the cathode, high catalytic activity of the cathode, and high electrical conductivity of the cathode. These factors ensure good operating performance of the cell or cell stack.
2. Description of the Prior Art
Presently, molten carbonate fuel cell components, anode, bubble barrier, cathode, and electrolyte materials are produced by a tape casting process in which the desired component material is mixed with a binder, a solvent, and other organic compounds to make a thick slurry, from which the component is cast. The binder remains after the solvent and other organic compounds evaporate. The anode, bubble barrier, and cathode, which are comprised primarily of metallic powders, are then heated to temperatures of about 800.degree. to 1100.degree. C. to sinter the powder material into a compact porous metallic structure, during which heating the binder is eliminated. The cell or cell stack is then assembled and freshly cast tapes of the electrolyte matrix and electrolyte carbonate are assembled into the individual cell or cell stack. These tapes are known as "green tapes" because they still contain the binder. The binder must be removed prior to melting of the electrolyte. Best results for binder removal have been obtained using air or an oxygen-containing gas at temperatures of up to about 450.degree. C. However, this results in premature oxidation of the metallic cathode. It is necessary that oxidation of the cathode take place after the electrolyte melts, otherwise high cell resistance results as a consequence of low lithium content in the lithiated nickel oxide cathode produced in the final oxidation process. It is most desireable that oxidation of the cathode occur only after it is wet with carbonate electrolyte, such as lithium-containing electrolyte. Presently, heat-up processes lasting up to seven to eight days are required for elimination of the binder and generally comprise heating to about 250.degree. to 300.degree. C., usually about 250.degree. C., using air or an oxygen-containing gas, since at those temperatures little oxidation of the nickel-containing cathode results. Above about 250.degree. to about 500.degree. C. an inert gas, generally nitrogen, is used during heating to prevent premature oxidation of the cathode. After the carbonate electrolyte melts at about 485.degree. C., regular process oxidants are commenced. However, gases from the decomposing binder and oxygen impurities in the inert nitrogen can be sufficiently high to produce premature oxidation of the metallic cathode. Further, undesired dry oxidation of the metallic cathode can occur after switching to oxidant gases if sufficient time has not been allowed for the carbonate electrolyte to soak into the cathode.
U.S. Pat. No. 4,810,595 teaches improvement or rejuvenation of voltage output during molten carbonate fuel cell operation by operating under load conditions with reduced or complete elimination of the flow of fuel and/or oxidant. Improved performance is observed after the active gas flow is restored, due to improved wetting of the electrodes. As recognized by the `595 patent, such improved cell output is observed only when the cell has fallen to low performance and the improvement is followed by subsequent decrease in cell operating performance.
U.S. Pat. No. 4,317,866 teaches use of high purity ceria as an anode material to provide oxidation resistance. U.S. Pat. No. 3,544,374 teaches hydroxide electrolyte fuel cells having a solid, hydrogen permeable metallic membrane anode. During start-up and shut-down corrosion or deterioration of the membrane is reduced by removal of hydrogen from the area of the metallic membrane and application of a negative DC potential to the anode making the anode negative with respect to the cathode and chemically inactive with respect to the electrolyte.